The present invention relates to a cathode ray tube and more particularly to improvements in its convergence regulator.
Cathode ray tubes are used for displaying images by deflecting electron beams from an electron gun and irradiating a phosphor screen with the electron beams. The formation of such an image on the phosphor screen is dependent on the intensity of the electron beams during the process of deflecting them.
Therefore, the formation of a reliable image is based on the deflection of electron beams with extreme precision and it is set forth as a premise that spots on a phosphor screen to be irradiated with the electron beams have to be precisely irradiated therewith.
Particularly in the case of a color cathode ray tube in which it has to be arranged that three electron beams from electron guns are directed to adjoining phosphors of three colors for use in different color development, spots on a phosphor screen to be irradiated with the electron beams need all the more precise irradiation.
If one of the electron beams fails to irradiate the target phosphor, color misregistration will occur.
Consequently, a convergence regulator is provided on the outer periphery of a so-called neck portion of the envelope for containing the electron guns of a cathode ray tube. With the provision of the convergence regulator, each electron beam is finely regulated in such a manner that a predetermined spot on a phosphor screen is precisely irradiated with each electron beam.
As disclosed in Japanese Patent Laid-Open No. 250335/1995 (corresponding European Patent Application No. 94114446.1), further, high magnetic permeability members are arranged in a neck portion so as to prevent electron beams from being affected by terrestrial magnetism, whereby highly reliable convergence regulation is attainable.
FIG. 8 is a sectional view of a neck portion in the prior art, wherein there are shown magnet plates 21, a holder 22, a neck portion 23 and high magnetic permeability members 24.
Each of the high magnetic permeability members is made of a conductive metal material and is formed as a member adhering to the neck-side inner wall surface of a convergence regulator to be mounted on the neck portion. Consequently, the high magnetic permeability members are placed on the interface between the convergence regulator and the neck portion.
In a cathode ray tube of the sort in which the high magnetic permeability members closely adhere to the neck portion, however, the withstand voltage of the neck portion of the envelope is insufficient.
Since a voltage as high as approximately 8-15 kV is applied to the electrode of the electron gun structure enclosed in the neck portion, the inner wall surface of the neck portion is charged with a voltage at the same level as the level of what has been applied thereto.
On the other hand, the high magnetic permeability members are formed in direct contact with the outer peripheral face of the neck portion. For this reason, the inner wall surface charged with the high voltage and the high magnetic permeability members are arranged close to each other and the withstand voltage therebetween tends to become insufficient.
In case a scratch so tiny as to be invisible or latent exists in the glass material forming the neck portion, it is a matter of considerable concern that deterioration in the pressure resistance of the glass will be accelerated and in the worst case, the neck portion will be broken.